Liquid crystal display device, method for producing liquid crystal display device, and monomer material for retardation layer

ABSTRACT

The present invention provides a liquid crystal display device that includes a retardation layer with excellent thermal stability and suppresses a decrease in contrast ratio caused by scattering even when the retardation layer has been formed by polymerizing reactive monomers. The liquid crystal display device includes a pair of substrates and a liquid crystal layer held between the substrates. At least one of the substrates includes a retardation layer that contains a polymer of at least one type of monomer. The at least one type of monomer includes a photo-alignment monomer represented by a certain formula.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 toJapanese Patent Application No. 2018-043085 filed on Mar. 9, 2018, thecontents of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to liquid crystal display devices, methodsfor producing a liquid crystal display device, and monomer materials fora retardation layer.

Description of Related Art

Recently, techniques for forming a retardation layer in a liquid crystaldisplay device have been studied. For example, a method for forming aretardation layer by polymerizing reactive monomers having been alignedby an alignment layer is known (e.g., JP 2016-196666 A).

BRIEF SUMMARY OF THE INVENTION

The inventor studied on techniques for forming a retardation layer in aliquid crystal display device in order to achieve clear visibility ofdisplay images in bright places (e.g., under natural light).

FIG. 4 is a schematic cross-sectional view of an example of aconventional liquid crystal display device in which a retardation layeris formed by a method using an alignment layer. As shown in FIG. 4, aliquid crystal display device 101 includes, in the order from theviewing surface side to the back surface side, a first linear polarizingplate 110, a color filter substrate 120, an alignment film 140 a, aliquid crystal layer 130, an alignment film 140 b, a thin filmtransistor array substrate 150, and a second linear polarizing plate160.

The color filter substrate 120 includes, in the order from the viewingsurface side to the back surface side, a support substrate 121, a colorfilter/black matrix 122, an alignment layer 190, and a retardation layer123.

The retardation layer 123 is formed by the method as shown in FIG. 5A,FIG. 5B, and FIG. 5C, for example. FIG. 5A, FIG. 5B, and FIG. 5C areschematic cross-sectional views for illustrating a method for forming aretardation layer using an alignment layer.

First, as shown in FIG. 5A, the alignment layer 190 is formed on thesurface of the color filter/black matrix 122. The alignment layer 190 isthen subjected to alignment treatment such as rubbing treatment orphoto-alignment treatment. The alignment layer 190 is formed of, forexample, a polyimide (polyamic acid). Next, a monomer material(solution) containing reactive monomers is applied to the surface of thealignment layer 190 to form a film 123 a containing the reactivemonomers as shown in FIG. 5B. Then, the film 123 a is heated in order topolymerize the reactive monomers, whereby the film 123 a is cured.Finally, the retardation layer 123 as shown in FIG. 5C is formed.

However, studies by the inventor revealed that forming the retardationlayer 123 using the alignment layer 190 involves the following issues(A) to (D).

(A) Conventional reactive monomers align along the direction given bythe alignment treatment performed to the alignment layer 190 and areincapable of self-alignment.

(B) The alignment layer 190 is formed on only one side of the film 123 a(retardation layer 123) and thus the alignment control force thereof isinsufficient.

Accordingly, a film 123 a with a large thickness (e.g., about 1 μm)deteriorates the alignability of the reactive monomers to increase therandomness thereof.

(C) Deteriorated alignability and increased randomness of the reactivemonomers decrease the thermal stability. Specifically, the energy fordisturbing the alignment of the reactive monomers by heat exceeds theenergy for stabilizing the alignment, which further deteriorates thealignability of the reactive monomers. The retardation of theretardation layer 123 obtained by polymerizing the reactive monomers isthus reduced by baking for alignment film 140 a formation or tends to bechanged (reduced) by long-term use.

(D) Deteriorated alignability of the reactive monomers tends to increasescattering on the retardation layer 123, leading to a reduced contrastratio of the liquid crystal display device.

An alignment layer 190 having undergone rubbing treatment causes thereactive monomers to have a pre-tilt angle of at least about 1°. Thismay cause insufficient retardation of the retardation layer 123, or theretardation may be influenced by the viewing angle.

An alignment layer 190 having undergone photo-alignment treatment isgiven weak alignment control force to the retardation layer 123 and thusreduces the retardation of the retardation layer 123 with time. Theretardation layer 123 may have insufficient function after long-termuse.

The present invention was devised under the current situation in the artand aims to provide a liquid crystal display device that includes aretardation layer with excellent thermal stability and suppresses adecrease in contrast ratio caused by scattering even when theretardation layer has been formed by polymerizing reactive monomers; amethod for producing a liquid crystal display device suitable forproduction of the liquid crystal display device; and a monomer materialfor a retardation layer suitable for formation of the retardation layer.

The inventor made various studies on techniques for forming aretardation layer in a liquid crystal display device and found that useof a photo-alignment monomer whose molecules are aligned by polarizedlight irradiation as a reactive monomer for retardation layer formationachieves excellent alignability. The inventor thereby found a means forthe above issues to complete the present invention.

In other words, an aspect of the present invention may be a liquidcrystal display device including: a pair of substrates, and a liquidcrystal layer held between the substrates, at least one of thesubstrates including a retardation layer that contains a polymer of atleast one type of monomer, the at least one type of monomer including aphoto-alignment monomer represented by the following formula (1).

In the formula (1), P¹ and P² are the same as or different from eachother and each represent an acrylic group or a methacrylic group; Sp′and Sp² are the same as or different from each other and each represent—O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or adirect bond; A¹ and A² are the same as or different from each other andeach represent a C1-C12 linear, branched, or cyclic saturated alkylenegroup, a C1-C12 linear, branched, or cyclic unsaturated alkylene group,or a direct bond; and Z¹ represents —O—, —S—, —NH—, —COO—, —COC—,—NHCO—, —CONH—, —NHCS—, —CSNH—, or a direct bond.

Another aspect of the present invention may be a method for producing aliquid crystal display device that includes a liquid crystal layer and asubstrate with a retardation layer, the method including: forming a filmthat contains at least one type of monomer including a photo-alignmentmonomer represented by the following formula (1), and irradiating thefilm with polarized light to align and polymerize molecules of thephoto-alignment monomer and thereby forming the retardation layer.

In the formula (1), P¹ and P² are the same as or different from eachother and each represent an acrylic group or a methacrylic group; Sp′and Sp² are the same as or different from each other and each represent—O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or adirect bond; A¹ and A² are the same as or different from each other andeach represent a C1-C12 linear, branched, or cyclic saturated alkylenegroup, a C1-C12 linear, branched, or cyclic unsaturated alkylene group,or a direct bond; and Z¹ represents —O—, —S—, —NH—, —COO—, —OCO—,—NHCO—, —CONH—, —NHCS—, —CSNH—, or a direct bond.

Still another aspect of the present invention may be a monomer materialfor a retardation layer, containing at least one type of monomerincluding a photo-alignment monomer represented by the following formula(1).

In the formula (1), P¹ and P² are the same as or different from eachother and each represent an acrylic group or a methacrylic group; Sp′and Sp² are the same as or different from each other and each represent—O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or adirect bond; A¹ and A² are the same as or different from each other andeach represent a C1-C12 linear, branched, or cyclic saturated alkylenegroup, a C1-C12 linear, branched, or cyclic unsaturated alkylene group,or a direct bond; and Z¹ represents —O—, —S—, —NH—, —COO—, OCO—, —NHCO—,—CONH—, —NHCS—, —CSNH—, or a direct bond.

The present invention can provide a liquid crystal display device thatincludes a retardation layer with excellent thermal stability andsuppresses a decrease in contrast ratio caused by scattering even whenthe retardation layer has been formed by polymerizing reactive monomers;a method for producing a liquid crystal display device suitable forproduction of the liquid crystal display device; and a monomer materialfor a retardation layer suitable for formation of the retardation layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a liquid crystal displaydevice of Embodiment 1.

FIG. 2A and FIG. 2B are schematic cross-sectional views for illustratinga method for forming a retardation layer using a photo-alignmentmonomer.

FIG. 3 is a schematic cross-sectional view of a liquid crystal displaydevice of Embodiment 2.

FIG. 4 is a schematic cross-sectional view of an example of aconventional liquid crystal display device in which a retardation layeris formed by a method using an alignment layer.

FIG. 5A, FIG. 5B, and FIG. 5C are schematic cross-sectional views forillustrating a method for forming a retardation layer using an alignmentlayer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in more detail based on embodimentswith reference to the drawings. The embodiments, however, are notintended to limit the present invention. The configuration of eachembodiment may appropriately be combined with each other or modifiedwithin the spirit of the present invention.

The expression “X to Y” herein means “X or more and Y or less”.

Embodiment 1

FIG. 1 is a schematic cross-sectional view of a liquid crystal displaydevice of Embodiment 1. As shown in FIG. 1, a liquid crystal displaydevice la includes, in the order from the viewing surface side to theback surface side, a first linear polarizing plate 10, a color filtersubstrate 20, a liquid crystal layer 30, an alignment film 40, a thinfilm transistor array substrate 50, and a second linear polarizing plate60.

The “viewing surface side” herein means the side closer to the screen(display surface) of a liquid crystal display device and is, forexample, the upper side (first linear polarizing plate 10 side) of theliquid crystal display device 1 a in FIG. 1. The “back surface side”herein means the side remote from the screen (display surface) of aliquid crystal display device and is, for example, the lower side(second linear polarizing plate 60 side) of the liquid crystal displaydevice 1 a in FIG. 1.

<First Linear Polarizing Plate and Second Linear Polarizing Plate>

Examples of the first linear polarizing plate 10 and the second linearpolarizing plate 60 include a polarizer (absorptive polarizing plate)formed by dyeing a polyvinyl alcohol film with an anisotropic materialsuch as an iodine complex (or dye) to adsorb the anisotropic material onthe film and then stretch-aligning the film. In order to ensure themechanical strength and the moisture and heat resistance, each side ofthe polyvinyl alcohol film is typically covered with a protective filmsuch as a triacetyl cellulose film for practical use.

The transmission axis of the first linear polarizing plate 10 and thetransmission axis of the second linear polarizing plate 60 arepreferably perpendicular to each other. This allows the first linearpolarizing plate 10 and the second linear polarizing plate 60 to bearranged in the crossed Nicols, which effectively achieves black displaywith no voltage applied to the liquid crystal layer 30 and gray-scaledisplay (e.g., intermediate gray-scale display, white display) withvoltage applied to the liquid crystal layer 30. The expression two axesare perpendicular to each other herein means the angle formed by the twoaxes is 87° to 93°, preferably 89° to 91°, more preferably 89.5° to90.5°, particularly preferably 90° (perfectly perpendicular with eachother).

<Color Filter Substrate>

The color filter substrate 20 includes, in the order from the viewingsurface side to the back surface side, a support substrate 21, a colorfilter/black matrix 22, and a retardation layer 23.

Examples of the support substrate 21 include a transparent substratesuch as a glass substrate or a plastic substrate.

The color filter/black matrix 22 includes red color filters, green colorfilters, and blue color filters in a plane, and the individual colorfilters are partitioned by a black matrix. The color filter/black matrix22 may be coated with an overcoat layer (transparent resin) thatfunctions as a flattening layer.

Examples of the material for the red color filters, green color filters,blue color filters, and black matrix include a resin (color resist)containing a pigment. The color combination of the color filter is notlimited to the combination of red, green, and blue, and may be acombination of red, green, blue, and yellow, for example.

The retardation layer 23 includes a polymer of at least one type ofmonomer. The retardation layer 23, which is disposed on the side closerto the liquid crystal layer 30 than the support substrate 21 (betweenthe support substrate 21 and the liquid crystal layer 30), is alsoreferred to as an in-cell retardation layer.

The “retardation layer” herein means a retardation layer that gives anin-plane retardation of 10 nm or longer to at least light with awavelength of 550 nm. The “retardation” herein means an in-planeretardation to light with a wavelength of 550 nm, unless otherwisespecified. Light with a wavelength of 550 nm is light having awavelength at which the human visual sensitivity is highest. Thein-plane retardation Re of a retardation layer is defined by theformula: Re=(ns−nf)×d. In the formula, ns represents nx or ny, whicheveris greater, and nf represents nx or ny, whichever is smaller, wherein nxand ny each represent a principal refractive index in an in-planedirection of the retardation layer, and d represents the thickness ofthe retardation layer. The principal refractive index is a value forlight with a wavelength of 550 nm unless otherwise specified. In theretardation layer, the in-plane slow axis is the axis in the directioncorresponding to ns and the in-plane fast axis is the axis in thedirection corresponding to nf. The retardation of the retardation layer23 is defined by Δn x d, i.e., the product of the birefringence Δn ofthe polymer forming the retardation layer 23 and the thickness d of theretardation layer 23.

The at least one type of monomer forming the polymer in the retardationlayer 23 includes a photo-alignment monomer represented by the formula(1).

In the formula (1), P¹ and P² are the same as or different from eachother and each represent an acrylic group or a methacrylic group. Sp¹and Sp² are the same as or different from each other and each represent—O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or adirect bond. A¹ and A² are the same as or different from each other andeach represent a C1-C12 linear, branched, or cyclic saturated alkylenegroup, a C1-C12 linear, branched, or cyclic unsaturated alkylene group,or a direct bond. Z¹ represents —O—, —S—, —NH—, —COO—, —OCO—, —NHCO—,—CONH—, —NHCS—, —CSNH—, or a direct bond.

The photo-alignment monomer represented by the formula (1) (hereinafter,also simply referred to as “photo-alignment monomer”) is a reactivemonomer that contains a mesogenic moiety in a molecule, i.e., a reactivemesogen monomer, and shows alignability by polarized light irradiation.Specifically, the photo-alignment monomer contains a coumarin group as aphoto-functional group, and thus molecules thereof are capable ofself-alignment. The coumarin group shows alignability by absorbingpolarized UV light, for example. Thus, there is no need to dispose analignment layer for aligning photo-alignment monomers.

Such photo-alignment monomers are aligned in the polarizedlight-irradiated direction. This enables uniform alignment in thethickness direction of the retardation layer 23 regardless of thethickness of the retardation layer 23. Use of photo-alignment monomersthus more improves the alignability in the entire retardation layer 23than the conventional case of providing an alignment layer on only oneside of the retardation layer to control the alignment of reactivemonomers. This allows the retardation of the retardation layer 23 to beless likely to change (decrease) even after long-term use and improvesthe thermal stability. This also improves the alignability of themesogenic moiety in the retardation layer 23, thereby suppressing adecrease in contrast ratio caused by scattering. Moreover, thephoto-alignment monomer contains, as shown in the formula (1), aflexible 4,4′-ethylene dianiline group with liquid crystallinity as wellas a rigid coumarin group functioning as a photo-functional group. Thisstructure achieves sufficiently stabilized alignability of thephoto-alignment monomer, leading to significantly improved thermalstability of the retardation layer 23.

Preferred specific examples of the photo-alignment monomer include thoserepresented by the following formula (2-1), (2-2), (2-3), (2-4), (2-5),(2-6), (2-7), (2-8), (2-9), (2-10), (2-11), or (2-12).

In the formulas (2-3), (2-4), (2-5), (2-6), (2-9), (2-10), (2-11), and(2-12), n is an integer of 1 to 12.

Preferred specific examples of the photo-alignment monomer representedby the formula (2-1), (2-2), (2-3), (2-4), (2-5), (2-6), (2-7), (2-8),(2-9), (2-10), (2-11), or (2-12) include those represented by thefollowing formula (3−1), (3−2), (3−3), (3−4), (3−5), (3−6), (3−7),(3−8), (3−9), (3−10), (3−11), or (3−12).

In the formulas (3−3), (3−4), (3-5), (3-6), (3-9), (3-10), (3-11), and(3-12), n is an integer of 1 to 12.

As shown in the formulas (2-2), (2-3), (2-5), (2-6), (2-8), (2-9),(2-11), and (2-12) (formulas (3-2), (3-3), (3-5), (3-6), (3-8), (3-9),(3-11), and (3-12)), a photo-alignment monomer containing an amino groupand an amide group causes intermolecular hydrogen bonding to increasethe thermal stability of the retardation layer 23. An example ofintermolecular hydrogen bonding caused in a photo-alignment monomer isillustrated by the following formula (H).

The retardation layer 23 is preferably a λ/4 retardation layer thatgives an in-plane retardation of a wavelength of ¼ to at least lightwith a wavelength of 550 nm. Specifically, a λ/4 retardation layer thatgives an in-plane retardation of 100 to 176 nm is preferred. When theretardation layer 23 is a λ/4 retardation layer, the angle formed by thein-plane slow axis of the retardation layer 23 and the transmission axisof the first linear polarizing plate 10 is preferably 45°. This allowsthe combination of the retardation layer 23 and the first linearpolarizing plate 10 to function as a circular polarizing plate. Thecircular polarizing plate has an antireflection effect which suppressesinternal reflection of the liquid crystal display device 1 a, therebysignificantly improving the visibility of display images in brightplaces (e.g., under natural light). The expression that the angle formedby two axes (directions) is 45° herein means the angle formed by the twoaxes is 42° to 48°, preferably 44° to 46°, more preferably 44.5° to45.5°, particularly preferably 45°.

The retardation layer 23 may be formed by the method as shown in FIG. 2Aand FIG. 2B. FIG. 2A and FIG. 2B are schematic cross-sectional views forillustrating a method for forming a retardation layer using aphoto-alignment monomer.

First, a monomer material (solution) for a retardation layer containinga photo-alignment monomer is applied to the surface of the colorfilter/black matrix 22 to form a film 23 a containing thephoto-alignment monomer as shown in FIG. 2A.

Examples of the solvent used in preparation of the monomer material fora retardation layer include propylene glycol monomethyl ether acetate,toluene, ethyl benzene, ethylene glycol monomethyl ether, ethyleneglycol dimethyl ether, propylene glycol methyl ether, dibutyl ether,acetone, methyl ethyl ketone, ethanol, propanol, cyclohexane,cyclopentanone, methyl cyclohexane, tetrahydrofuran, dioxane,cyclohexanone, n-hexane, ethyl acetate, butyl acetate, methoxy butylacetate, N-methylpyrrolidone, and dimethyl acetamide. These solvents maybe used alone or in combination of two or more thereof.

Next, as shown in FIG. 2A, the film 23 a is irradiated with polarizedlight (e.g., polarized UV light) while being heated at a temperature notlower than the nematic-isotropic phase transition temperature of thephoto-alignment monomer, whereby molecules of the photo-alignmentmonomer are aligned and polymerized. As a result, the film 23 a is curedinto the retardation layer 23 as shown in FIG. 2B.

In the curing of the film 23 a, the polarized light irradiation may befollowed by baking (heat treatment) for completing the polymerization ofthe photo-alignment monomer and removing the solvent.

The surface of the retardation layer 23 may be subjected to rubbingtreatment. This causes the retardation layer 23 to exhibit alignmentcontrol force to liquid crystal molecules in the liquid crystal layer30.

In the retardation layer 23, molecules of the photo-alignment monomerare uniformly aligned in the entire layer as well as on the surface.Accordingly, rubbing treatment to the surface of the retardation layer23 causes no reduction in retardation due to disturbance of thealignment of molecules of the photo-alignment monomer. The rubbingtreatment to the retardation layer 23 is preferably performed in the 45°direction with respect to the in-plane slow axis of the retardationlayer 23.

<Thin Film Transistor Array Substrate>

The thin film transistor array substrate 50 may be, for example, anactive matrix substrate typically used in the field of liquid crystaldisplay devices. In a fringe field switching (FFS)-mode liquid crystaldisplay device 1 a, the thin film transistor array substrate 50includes, for example, a support substrate, a common electrode (planarelectrode) disposed on the liquid crystal layer 30 side surface of thesupport substrate, an insulating film covering the common electrode, andpixel electrodes (slit electrodes) disposed on the liquid crystal layer30 side surface of the insulating film. Here, when voltage is appliedbetween the common electrode and the pixel electrodes (with voltageapplied), a transverse electric field (fringe electric field) isgenerated in the liquid crystal layer 30, which controls the alignmentof liquid crystal molecules in the liquid crystal layer 30. In anin-plane switching (IPS)-mode liquid crystal display device 1 a, whenvoltage is applied between paired comb electrodes disposed on the thinfilm transistor array substrate 50 (with voltage applied), a transverseelectric field is generated in the liquid crystal layer 30, whichcontrols the alignment of liquid crystal molecules in the liquid crystallayer 30.

<Alignment Film>

The alignment film 40 may be a horizontal alignment film typically usedin the field of liquid crystal display devices, for example. Ahorizontal alignment film aligns liquid crystal molecules in the liquidcrystal layer 30 in the direction parallel to its surface. Theexpression that liquid crystal molecules are aligned in the directionparallel to the surface of the horizontal alignment film (horizontallyaligned) herein means that the pre-tilt angle of liquid crystalmolecules is 0° to 5°, preferably 0° to 2°, more preferably 0° to 1°,with respect to the surface of the horizontal alignment film. Thepre-tilt angle of a liquid crystal molecule is the angle of the majoraxis of the liquid crystal molecule inclining to the surface of thehorizontal alignment film with no voltage applied to the liquid crystallayer 30. The alignment film 40 may have undergone alignment treatmentsuch as rubbing treatment or photo-alignment treatment. In other words,the alignment film 40 may be an alignment film for rubbing treatment ora photo-alignment film.

<Liquid Crystal Layer>

The liquid crystal material constituting the liquid crystal layer 30 maybe a positive liquid crystal material having positive anisotropy ofdielectric constant or a negative liquid crystal material havingnegative anisotropy of dielectric constant. In a liquid crystal displaydevice 1 a of the transverse electric field mode such as the FFS mode orthe IPS mode, when the surface of the retardation layer 23 has undergonerubbing treatment and the alignment film 40 is a horizontal alignmentfilm, the alignment control force of the retardation layer 23 and thealignment film 40 horizontally aligns liquid crystal molecules in theliquid crystal layer 30 at a predetermined azimuth with no voltageapplied to the liquid crystal layer 30. Meanwhile, liquid crystalmolecules in the liquid crystal layer 30, with voltage applied to theliquid crystal layer 30, rotate in an in-plane direction in response tothe transverse electric field generated in the liquid crystal layer 30.

The liquid crystal display device 1 a may further include anantireflection film on the viewing surface side (side opposite to thecolor filter substrate 20) of the first linear polarizing plate 10. Thissuppresses the surface reflection of the liquid crystal display device 1a. The antireflection film is preferably an optical film having asurface with a moth-eye structure (structure of a moth's eye).

The liquid crystal display device 1 a may further include a backlight onthe back surface side (side opposite from the thin film transistor arraysubstrate 50) of the second linear polarizing plate 60. In this case,the liquid crystal display device 1 a is a transmissive liquid crystaldisplay device.

The liquid crystal display device 1 a may further include, in additionto the described members, members typically used in the field of liquidcrystal display devices, e.g., external circuits such as a tape carrierpackage (TCP) and a print circuit board (PCB); and a bezel (frame), asappropriate.

Embodiment 2

FIG. 3 is a schematic cross-sectional view of a liquid crystal displaydevice of Embodiment 2. As shown in FIG. 3, a liquid crystal displaydevice 1 b includes, in the order from the viewing surface side to theback surface side, the first linear polarizing plate 10, the colorfilter substrate 20, a photo-alignment film 70, the liquid crystal layer30, the alignment film 40, the thin film transistor array substrate 50,and the second linear polarizing plate 60. Since the members excludingthe photo-alignment film 70 of the liquid crystal display device 1 b areas described above, the description thereof is omitted.

<Photo-Alignment Film>

The photo-alignment film 70 contains a material with photo-alignabilityand can exert alignment control force to liquid crystal molecules, i.e.,can control the alignment of liquid crystal molecules, in the liquidcrystal layer 30 by light irradiation. In the liquid crystal displaydevice 1 b, the photo-alignment film 70 is disposed on the liquidcrystal layer 30 side of the retardation layer 23. This improves thealignability of liquid crystal molecules in the liquid crystal layer 30and thereby increases the contrast ratio of the liquid crystal displaydevice 1 b. The photo-alignment film 70 may function as a horizontalalignment film (horizontal photo-alignment film).

The expression “material with photo-alignability” herein means a generalmaterial that undergoes structural change by light irradiation such asUV light or visible light irradiation and thereby controls the alignmentof (exerts alignment control force to) nearby liquid crystal moleculesor changes the intensity and/or direction of the alignment controlforce.

Examples of the material with photo-alignability include materials witha photo-functional group (photo-reactive moiety) that causes a reactionsuch as photodimerization, photoisomerization, photo Friesrearrangement, or photolysis by light irradiation. Examples of aphoto-functional group capable of photodimerization andphotoisomerization include cinnamate, chalcone, coumarin, and stilbenegroups. Examples of a photo-functional group capable ofphotoisomerization include an azobenzene group. Examples of aphoto-functional group capable of photo Fries rearrangement include aphenol ester group. Examples of a photo-functional group capable ofphotolysis include a cyclobutane ring.

The photo-alignment film 70 disposed with the retardation layer 23 ispreferably a photo-alignment film that contains a polymer containing acinnamate group or a photo-alignment film that contains a polymer with astructure derived from a cyclobutane ring. Particularly, when thephoto-alignment film 70 contains a polymer containing a cinnamate group,an exciplex is formed between a cinnamate skeleton portion of thephoto-alignment film 70 and a coumarin skeleton portion of theretardation layer 23, which improves the thermal stability between thephoto-alignment film 70 and the retardation layer 23.

The photo-alignment film that contains a polymer containing a cinnamategroup may be formed by the following method. First, an alignment filmmaterial that contains a polymer containing a cinnamate group (materialwith photo-alignability) is applied to the surface of the retardationlayer 23 to form a polymer film that contains the polymer containing acinnamate group. This polymer film is irradiated with polarized light(e.g., polarized UV light) to exert alignment control force to liquidcrystal molecules in the liquid crystal layer 30.

The photo-alignment film that contains a polymer with a structurederived from a cyclobutane ring may be formed by the following method.First, an alignment film material that contains a polymer containing acyclobutane ring (material with photo-alignability) is applied to thesurface of the retardation layer 23 to form a polymer film that containsthe polymer containing a cyclobutane ring. This polymer film isirradiated with polarized light (e.g., polarized UV light) to exertalignment control force to liquid crystal molecules in the liquidcrystal layer 30.

Although the retardation layer 23 is disposed only in the color filtersubstrate 20 in Embodiments 1 and 2, the retardation layer 23 may bedisposed only in the thin film transistor array substrate 50 or in bothof the color filter substrate 20 and the thin film transistor arraysubstrate 50.

EXAMPLES AND COMPARATIVE EXAMPLES

The present invention is described below in more detail based onexamples and comparative examples. The examples, however, are notintended to limit the scope of the present invention. In the followingdescription regarding formation of a retardation layer, a laminateincluding a support substrate and a color filter/black matrix may alsobe referred to as a color filter substrate for convenience.

Example 1

A liquid crystal display device of Embodiment 1 as a liquid crystaldisplay device of Example 1 was produced by the following method.

First, a monomer material for a retardation layer was prepared bydissolving 8 wt % of a photo-alignment monomer M1 represented by thefollowing formula (3-1) in propylene glycol monomethyl ether acetate asa solvent.

The monomer material for a retardation layer was applied to a surface ofa color filter substrate (color filter/black matrix) with a spin coaterto form a film containing the photo-alignment monomer M1.

The color filter substrate with the film containing the photo-alignmentmonomer M1 was placed on a hot plate at 140° C. for one minute to bepre-baked. The film containing the photo-alignment monomer M1 was thenirradiated with polarized UV light (central wavelength: 365 nm,irradiation dose: 5 J/cm²), whereby molecules of the photo-alignmentmonomer M1 were aligned and polymerized. Subsequently, post-baking wasperformed at 180° C. for 30 minutes to complete the polymerization ofthe photo-alignment monomer M1 and perfectly remove the solvent. As aresult, the film containing the photo-alignment monomer M1 wascompletely cured into a retardation layer. The retardation layer had athickness of 1 μm and a retardation of 135 nm.

Next, a thin film transistor array substrate for the FFS mode includingpixel electrodes and a common electrode and a color filter substrateincluding the retardation layer and no electrode were prepared. Then, amaterial for a polyimide alignment film for rubbing treatment wasapplied to a surface of the thin film transistor array substrate to forma film, and the surface of the film was subjected to rubbing treatment.As a result, an alignment film for rubbing treatment was formed on thesurface of the thin film transistor array substrate. Meanwhile, thesurface of the retardation layer of the color filter substrate wassubjected to rubbing treatment such that the rubbing direction wasantiparallel to the rubbing direction for the alignment film for rubbingtreatment on the thin film transistor array substrate side (antiparallelalignment). The rubbing treatment to the retardation layer was performedin the 45° direction with respect to the in-plane slow axis of theretardation layer.

Next, a seal material was applied in a pattern to the surface of thethin film transistor array substrate. The seal material used was a UVlight/heat curable seal material that is cured by both UV light andheat. Subsequently, a positive liquid crystal material(nematic-isotropic phase transition temperature: 90° C.) was dropped onthe surface of the thin film transistor array substrate (in the regionsurrounded by the seal material), and the substrate was bonded with thecolor filter substrate including the retardation layer. Thereby, aliquid crystal layer was formed in the region surrounded by the sealmaterial in a plan view. Then, the temperature of the liquid crystallayer was controlled at 100° C. (a temperature not lower than thenematic-isotropic phase transition temperature of the liquid crystalmaterial), whereby realignment treatment was performed. As a result, theliquid crystal display device of Example 1 that included a retardationlayer (FFS mode liquid crystal display device) was completed.

Example 2

A liquid crystal display device of Example 2 was produced in the samemanner as with the liquid crystal display device of Example 1 exceptthat a photo-alignment monomer M2 represented by the following formula(3-2) was used in place of the photo-alignment monomer M1.

Example 3

A liquid crystal display device of Example 3 was produced in the samemanner as with the liquid crystal display device of Example 1 exceptthat a photo-alignment monomer M3 represented by the following formula(3-3-1) was used in place of the photo-alignment monomer M1.

Comparative Example 1

A liquid crystal display device of Comparative Example 1 was produced bythe following method.

First, a monomer material for a retardation layer was prepared bydissolving 5 wt % of a photo-alignment monomer N1 represented by thefollowing formula (4) in propylene glycol monomethyl ether acetate as asolvent.

The monomer material for a retardation layer was applied to a surface ofa color filter substrate (color filter/black matrix) with a spin coaterto form a film containing the photo-alignment monomer N1.

The color filter substrate with the film containing the photo-alignmentmonomer N1 was placed on a hot plate at 140° C. for one minute to bepre-baked. The film containing the photo-alignment monomer N1 was thenirradiated with polarized UV light (central wavelength: 365 nm,irradiation dose: 5 J/cm²), whereby molecules of the photo-alignmentmonomer N1 were aligned and polymerized. Subsequently, post-baking wasperformed at 180° C. for 30 minutes to complete the polymerization ofthe photo-alignment monomer N1 and perfectly remove the solvent. As aresult, the film containing the photo-alignment monomer N1 wascompletely cured into a retardation layer. The retardation layer had athickness of 2 μm and a retardation of 135 nm.

Then, the same processes as for the liquid crystal display device ofExample 1 were taken except for using the color filter substrateincluding the retardation layer obtained by the described method,whereby the liquid crystal display device of Comparative Example 1 wasproduced.

<Evaluation 1>

The liquid crystal display devices of Examples 1 to 3 and ComparativeExample 1 were evaluated on the following properties. Table 1 shows theresults.

(Thermal Stability of Retardation Layer)

The retardation of the retardation layer was determined by theellipsometry technique before and after the baking performed by placingthe liquid crystal display device on a hot plate at 200° C. for 30minutes.

(Contrast Ratio)

The contrast ratio of the liquid crystal display device was measured ina dark room using a luminance meter “Topcon BM5” available from TopconTechnohouse Corporation.

(Outdoor Visibility)

The display region in the display state of the liquid crystal displaydevice was equivalently divided into nine regions (the regions may bearranged in a longitudinal or lateral direction) consisting of “a regionproviding 0 gray-scale display”, “a region providing 32 gray-scaledisplay”, “a region providing 64 gray-scale display”, “a regionproviding 96 gray-scale display”, “a region providing 128 gray-scaledisplay”, “a region providing 160 gray-scale display”, “a regionproviding 192 gray-scale display”, “a region providing 224 gray-scaledisplay”, and “a region providing 255 gray-scale display”. The displayregion was observed outdoors under sunlight, and whether the differencesbetween the gray-scale displays were recognized or not was determined.The evaluation criteria were as follows.

Good: The differences between the gray-scale displays were recognized.

Poor: No difference between the gray-scale displays was recognized.

TABLE 1 Retardation of retardation layer (nm) Contrast Outdoor Beforebaking After baking ratio visibility Example 1 135 123 525 Good Example2 135 129 535 Good Example 3 135 129 535 Good Comparative 135 103 480Good Example 1

In Examples 1 to 3, the decrease in retardation of the retardation layerfrom before to after the baking was small, i.e., the thermal stabilityof the retardation layer was excellent. The photo-alignment monomers M1,M2, and M3 used in Examples 1, 2, and 3, respectively, each included aflexible 4,4′-ethylene dianiline group with liquid crystallinity inaddition to a coumarin group functioning as a photo-functional group andcapable of self-alignment. This achieved uniform alignment in the entireretardation layer (particularly, in the thickness direction), whichpresumably suppressed the decrease in retardation caused by heat.Furthermore, the photo-alignment monomers M2 and M3 used in Examples 2and 3, respectively, each contained an amino group and an amide group.This caused a large amount of intermolecular hydrogen bonding, whichpresumably more improved the thermal stability of the retardation layer.

In Examples 1 to 3, the decrease in contrast ratio caused by scatteringon the retardation layer was suppressed and the outdoor visibility wasgood.

In Comparative Example 1, the decrease in retardation of the retardationlayer from before to after the baking was large, i.e., the thermalstability of the retardation layer was poor. Although containing acoumarin group, the photo-alignment monomer N1 used in ComparativeExample 1 was more rigid and had a lower solubility in the solvent thanthe photo-alignment monomers M1, M2, and M3, and thereby presumablyfailed in providing a retardation layer excellent in thermal stability.

Example 4

A liquid crystal display device of Embodiment 2 as a liquid crystaldisplay device of Example 4 was produced by the following method.

First, a monomer material for a retardation layer was prepared bydissolving 10 wt % of a photo-alignment monomer M4 represented by thefollowing formula (3-4-1) in propylene glycol monomethyl ether acetateas a solvent.

The monomer material for a retardation layer was applied to a surface ofa color filter substrate (color filter/black matrix) with a spin coaterto form a film containing the photo-alignment monomer M4.

The color filter substrate with the film containing the photo-alignmentmonomer M4 was placed on a hot plate at 140° C. for one minute to bepre-baked. The film containing the photo-alignment monomer M4 was thenirradiated with polarized UV light (central wavelength: 365 nm,irradiation dose: 5 J/cm²), whereby molecules of the photo-alignmentmonomer M4 were aligned and polymerized. Subsequently, post-baking wasperformed at 180° 0 for 30 minutes to complete the polymerization of thephoto-alignment monomer M4 and perfectly remove the solvent. As aresult, the film containing the photo-alignment monomer M4 wascompletely cured into a retardation layer. The retardation layer had athickness of 1 μm and a retardation of 135 nm.

Next, a thin film transistor array substrate for the FFS mode includingpixel electrodes and a common electrode and a color filter substrateincluding the retardation layer and no electrode were prepared. On asurface of each substrate, a polymer film that contained a polymercontaining a cinnamate group represented by the following formula (5)was formed. Subsequently, the polymer films on the respective substrateswere subjected to photo-alignment treatment to have alignmentsantiparallel to each other (antiparallel alignment). Thereby, aphoto-alignment film that contained a polymer containing a cinnamategroup was formed on the surface of each substrate. The photo-alignmenttreatment to each polymer film was performed by irradiation withpolarized UV light with a wavelength of 280 to 330 nm using a cutfilter. The photo-alignment treatment to the polymer film on the colorfilter substrate side was performed in the 45° direction with respect tothe in-plane slow axis of the retardation layer.

In the formula (5), p represents a polymerization degree.

Next, a seal material was applied in a pattern to the surface of thethin film transistor array substrate. The seal material used was a UVlight/heat curable seal material that is cured by either of UV light orheat. Subsequently, a positive liquid crystal material(nematic-isotropic phase transition temperature: 90° C.) was dropped onthe surface of the thin film transistor array substrate (in the regionsurrounded by the seal material), and the substrate was bonded with thecolor filter substrate including the retardation layer. Thereby, aliquid crystal layer was formed in the region surrounded by the sealmaterial in a plan view. Then, the temperature of the liquid crystallayer was controlled at 100° C. (a temperature not lower than thenematic-isotropic phase transition temperature of the liquid crystalmaterial), whereby realignment treatment was performed. As a result, theliquid crystal display device of Example 4 that included a retardationlayer (FFS mode liquid crystal display device) was completed.

Example 5

A liquid crystal display device of Example 5 was produced in the samemanner as with the liquid crystal display device of Example 4 exceptthat a polyimide alignment film for rubbing treatment was used in placeof the photo-alignment film.

Comparative Example 2

A liquid crystal display device of Comparative Example 2 was produced inthe same manner as with the liquid crystal display device of Example 4except that the retardation layer was not formed on the color filtersubstrate.

<Evaluation 2>

The liquid crystal display devices of Examples 4 and 5 and ComparativeExample 2 were evaluated in the same manner as in Evaluation 1. Table 2shows the results.

TABLE 2 Retardation of retardation layer (nm) Contrast Outdoor Beforebaking After baking ratio visibility Example 4 135 127 595 Good Example5 135 127 500 Good Comparative — — 900 Poor Example 2

In Examples 4 and 5, the decrease in retardation of the retardationlayer from before to after the baking was small, i.e., the thermalstability of the retardation layer was excellent. The photo-alignmentmonomer M4 used in Examples 4 and 5 included a flexible 4,4′-ethylenedianiline group with liquid crystallinity in addition to a coumaringroup functioning as a photo-functional group and capable ofself-alignment. This achieved uniform alignment in the entireretardation layer (particularly, in the thickness direction), whichpresumably suppressed the decrease in retardation caused by heat.Furthermore, in Example 4, an exciplex was formed between a cinnamateskeleton portion of the photo-alignment film and a coumarin skeletonportion of the retardation layer, which presumably more improved thethermal stability of the retardation layer.

In Examples 4 and 5, the decrease in contrast ratio caused by scatteringon the retardation layer was suppressed and the outdoor visibility wasgood. In particular, in Example 4 using a photo-alignment film, thecontrast ratio and the outdoor visibility were both better than inExample 5 using an alignment film for rubbing treatment. The bettercontrast ratio in Example 4 was presumably achieved by suppression ofscattering on the retardation layer, use of a photo-alignment film, andexciplex formation between a cinnamate skeleton portion of thephoto-alignment film and a coumarin skeleton portion of the retardationlayer, which improved the thermal stability of the retardation layer.

In Comparative Example 2 where no retardation layer was provided, theoutdoor visibility was poor.

Example 6

A liquid crystal display device of Embodiment 2 as a liquid crystaldisplay device of Example 6 was produced by the following method.

First, a monomer material for a retardation layer was prepared bydissolving 10 wt % of a photo-alignment monomer M5 represented by thefollowing formula (3-9-1) in propylene glycol monomethyl ether acetateas a solvent.

The monomer material for a retardation layer was applied to a surface ofa color filter substrate (color filter/black matrix) with a spin coaterto form a film containing the photo-alignment monomer M5.

The color filter substrate with the film containing the photo-alignmentmonomer M5 was placed on a hot plate at 140° C. for one minute to bepre-baked. The film containing the photo-alignment monomer M5 was thenirradiated with polarized UV light (central wavelength: 365 nm,irradiation dose: 5 J/cm²), whereby molecules of the photo-alignmentmonomer M5 were aligned and polymerized. Subsequently, post-baking wasperformed at 180° C. for 30 minutes to complete the polymerization ofthe photo-alignment monomer M5 and perfectly remove the solvent. As aresult, the film containing the photo-alignment monomer M5 wascompletely cured into a retardation layer. The retardation layer had athickness of 1 μm and a retardation of 135 nm.

Next, a thin film transistor array substrate for the FFS mode includingpixel electrodes and a common electrode and a color filter substrateincluding the retardation layer and no electrode were prepared. Then, ona surface of each substrate, a polymer film that contained a polymercontaining a cyclobutane ring represented by the following formula (6)was formed. Subsequently, the polymer films on the respective substrateswere subjected to photo-alignment treatment to have alignmentsantiparallel to each other (antiparallel alignment). Thereby, aphoto-alignment film that contained a polymer with a structure derivedfrom a cyclobutane ring was formed on the surface of each substrate. Thephoto-alignment treatment to each polymer film was performed byirradiation with polarized UV light with a wavelength not longer than300 nm using a cut filter. The photo-alignment treatment to the polymerfilm on the color filter substrate side was performed in the 45°direction with respect to the in-plane slow axis of the retardationlayer.

In the formula (6), p represents a polymerization degree.

Next, a seal material was applied in a pattern to the surface of thethin film transistor array substrate. The seal material used was a UVlight/heat curable seal material that is cured by either of UV light orheat. Subsequently, a negative liquid crystal material(nematic-isotropic phase transition temperature: 80° C.) was dropped onthe surface of the thin film transistor array substrate (in the regionsurrounded by the seal material), and the substrate was bonded with thecolor filter substrate including the retardation layer. Thereby, aliquid crystal layer was formed in the region surrounded by the sealmaterial in a plan view. Then, the temperature of the liquid crystallayer was controlled at 100° C. (a temperature not lower than thenematic-isotropic phase transition temperature of the liquid crystalmaterial), whereby realignment treatment was performed. As a result, theliquid crystal display device of Example 6 that included a retardationlayer (FFS mode liquid crystal display device) was completed.

Comparative Example 3

A liquid crystal display device of Comparative Example 3 was produced inthe same manner as with the liquid crystal display device of Example 6except that the retardation layer was not formed on the color filtersubstrate.

<Evaluation 3>

The liquid crystal display devices of Example 6 and Comparative Example3 were evaluated in the same manner as in Evaluation 1. Table 3 showsthe results.

TABLE 3 Retardation of retardation layer (nm) Contrast Outdoor Beforebaking After baking ratio visibility Example 6 135 127 580 GoodComparative — — 1000 Poor Example 3

In Example 6, the decrease in retardation of the retardation layer frombefore to after the baking was small, i.e., the thermal stability of theretardation layer was excellent. The photo-alignment monomer M5 used inExample 6 included a flexible 4,4′-ethylene dianiline group with liquidcrystallinity in addition to a coumarin group functioning as aphoto-functional group and capable of self-alignment. This achieveduniform alignment in the entire retardation layer (particularly, in thethickness direction), which presumably suppressed the decrease inretardation caused by heat. Furthermore, the photo-alignment monomer M5used in Example 6, which contained an amino group and an amide group,caused a large amount of intermolecular hydrogen bonding, whichpresumably more improved the thermal stability of the retardation layer.

In Example 6, the decrease in contrast ratio caused by scattering on theretardation layer was suppressed and the outdoor visibility was good.The better contrast ratio in Example 6 was presumably achieved bysuppression of scattering on the retardation layer, use of aphoto-alignment film, and use of a negative liquid crystal material,which improved the transmittance.

In Comparative Example 3 where no retardation layer was provided, theoutdoor visibility was poor.

[Additional Remarks]

A first aspect of the present invention may be a liquid crystal displaydevice including: a pair of substrates, and a liquid crystal layer heldbetween the substrates, at least one of the substrates including aretardation layer that contains a polymer of at least one type ofmonomer, the at least one type of monomer including a photo-alignmentmonomer represented by the following formula (1). This aspect achieves aliquid crystal display device that includes the retardation layer withexcellent thermal stability and suppresses a decrease in contrast ratiocaused by scattering.

In the formula (1), P¹ and P² are the same as or different from eachother and each represent an acrylic group or a methacrylic group; Sp¹and Sp² are the same as or different from each other and each represent—O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or adirect bond; A¹ and A² are the same as or different from each other andeach represent a C1-C12 linear, branched, or cyclic saturated alkylenegroup, a C1-C12 linear, branched, or cyclic unsaturated alkylene group,or a direct bond; and

Z¹ represents —O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—, —NHCS—,—CSNH—, or a direct bond.

In the first aspect of the present invention, the photo-alignmentmonomer may be represented by the following formula (2-1), (2-2), (2-3),(2-4), (2-5), (2-6), (2-7), (2-8), (2-9), (2-10), (2-11), or (2-12).Specifically, the photo-alignment monomer may be represented by thefollowing formula (3-1), (3-2), (3-3), (3-4), (3-5), (3-6), (3-7),(3-8), (3-9), (3-10), (3-11), or (3-12). This allows the photo-alignmentmonomer to effectively function in the retardation layer.

In the formulas (2-3), (2-4), (2-5), (2-6), (2-9), (2-10), (2-11), and(2-12), n is an integer of 1 to 12.

In the formulas (3-3), (3-4), (3-5), (3-6), (3-9), (3-10), (3-11), and(3-12), n is an integer of 1 to 12.

In the first aspect of the present invention, the liquid crystal displaydevice may further include, between at least one of the substrates andthe liquid crystal layer, an alignment film that controls alignment ofliquid crystal molecules in the liquid crystal layer. The alignment filmmay be a horizontal alignment film that aligns the liquid crystalmolecules in the liquid crystal layer in a direction parallel to asurface of the alignment film. This allows liquid crystal molecules inthe liquid crystal layer to horizontally align at a predeterminedazimuth.

In the first aspect of the present invention, the alignment film may bea photo-alignment film. The photo-alignment film may contain a polymercontaining a cinnamate group or a polymer with a structure derived froma cyclobutane ring. This improves the alignability of liquid crystalmolecules in the liquid crystal layer, leading to an increase incontrast ratio of the liquid crystal display device. In particular, whenthe photo-alignment film contains a polymer containing a cinnamategroup, an exciplex is formed between a cinnamate skeleton portion of thephoto-alignment film and a coumarin skeleton portion of the retardationlayer, which improves the thermal stability between the photo-alignmentfilm and the retardation layer.

A second aspect of the present invention may be a method for producing aliquid crystal display device that includes a liquid crystal layer and asubstrate with a retardation layer, the method including: forming a filmthat contains at least one type of monomer including a photo-alignmentmonomer represented by the following formula (1), and irradiating thefilm with polarized light to align and polymerize molecules of thephoto-alignment monomer and thereby forming the retardation layer. Thisaspect achieves a method for producing a liquid crystal display devicethat includes the retardation layer with excellent thermal stability andsuppresses a decrease in contrast ratio caused by scattering.

In the formula (1), 2¹ and p² are the same as or different from eachother and each represent an acrylic group or a methacrylic group; Sp¹and Sp² are the same as or different from each other and each represent—O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or adirect bond; A¹ and A² are the same as or different from each other andeach represent a C1-C12 linear, branched, or cyclic saturated alkylenegroup, a C1-C12 linear, branched, or cyclic unsaturated alkylene group,or a direct bond; and Z¹ represents —O—, —S—, —NH—, —COO—, —OCO—,—NHCO—, —CONH—, —NHCS—, —CSNH—, or a direct bond.

In the second aspect of the present invention, the method may furtherinclude subjecting a surface of the retardation layer to rubbingtreatment and thereby causing the retardation layer to exhibit alignmentcontrol force to liquid crystal molecules in the liquid crystal layer.This can prevent a decrease in retardation caused by disorderedalignment of the photo-alignment monomer molecules to cause alignment ofliquid crystal molecules in the liquid crystal layer.

In the second aspect of the present invention, the method may furtherinclude forming on a surface of the retardation layer a polymer filmthat contains a polymer containing a cinnamate group, and irradiatingthe polymer film with polarized light and thereby causing the polymerfilm to exhibit alignment control force to liquid crystal molecules inthe liquid crystal layer. This allows formation of a photo-alignmentfilm that contains a polymer containing a cinnamate group on the liquidcrystal layer side of the retardation layer to improve the alignabilityof liquid crystal molecules in the liquid crystal layer, leading to anincrease in contrast ratio of the liquid crystal display device. Inaddition, an exciplex is formed between a cinnamate skeleton portion ofthe photo-alignment film and a coumarin skeleton portion of theretardation layer, which improves the thermal stability between thephoto-alignment film and the retardation layer.

In the second aspect of the present invention, the method may furtherinclude forming on a surface of the retardation layer a polymer filmthat contains a polymer containing a cyclobutane ring, and irradiatingthe polymer film with polarized light and thereby causing the polymerfilm to exhibit alignment control force to liquid crystal molecules inthe liquid crystal layer. This allows formation of a photo-alignmentfilm that contains a polymer with a structure derived from a cyclobutanering on the liquid crystal layer side of the retardation layer toimprove the alignability of liquid crystal molecules in the liquidcrystal layer, leading to an increase in contrast ratio of the liquidcrystal display device.

A third aspect of the present invention may be a monomer material for aretardation layer, containing at least one type of monomer including aphoto-alignment monomer represented by the following formula (1). Thisaspect achieves a monomer material for a retardation layer suitable forformation of a retardation layer excellent in thermal stability.

In the formula (1), P¹ and P² are the same as or different from eachother and each represent an acrylic group or a methacrylic group; Sp¹and Sp² are the same as or different from each other and each represent—O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or adirect bond; A¹ and A² are the same as or different from each other andeach represent a C1-C12 linear, branched, or cyclic saturated alkylenegroup, a C1-C12 linear, branched, or cyclic unsaturated alkylene group,or a direct bond; and Z¹ represents —O—, —S—, —NH—, —COO—, —OCO—,—NHCO—, —CONH—, —NHCS—, —CSNH—, or a direct bond.

What is claimed is:
 1. A liquid crystal display device comprising: apair of substrates, and a liquid crystal layer held between thesubstrates, at least one of the substrates including a retardation layerthat contains a polymer of at least one type of monomer, the at leastone type of monomer including a photo-alignment monomer represented bythe following formula (1):

wherein P¹ and P² are the same as or different from each other and eachrepresent an acrylic group or a methacrylic group; Sp¹ and Sp² are thesame as or different from each other and each represent —O—, —S—, —NH—,—COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or a direct bond; A¹ andA² are the same as or different from each other and each represent aC1-C12 linear, branched, or cyclic saturated alkylene group, a C1-C12linear, branched, or cyclic unsaturated alkylene group, or a directbond; and Z^(I) represents —O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—,—NHCS—, —CSNH—, or a direct bond.
 2. The liquid crystal display deviceaccording to claim 1, wherein the photo-alignment monomer is representedby the following formula (2-1), (2-2), (2-3), (2-4), (2-5), (2-6),(2-7), (2-8), (2-9), (2-10), (2-11), or (2-12):

wherein, in the formulas (2-3), (2-4), (2-5), (2-6), (2-9), (2-10),(2-11), and (2-12), n is an integer of 1 to
 12. 3. The liquid crystaldisplay device according to claim 2, wherein the photo-alignment monomeris represented by the following formula (3-1), (3-2), (3-3), (3-4),(3-5), (3-6), (3-7), (3-8), (3-9), (3-10), (3-11), or (3-12):

wherein, in the formulas (3-3), (3-4), (3-5), (3-6), (3-9), (3-10),(3-11), and (3-12), n is an integer of 1 to
 12. 4. The liquid crystaldisplay device according to claim 1, wherein the liquid crystal displaydevice further includes, between at least one of the substrates and theliquid crystal layer, an alignment film that controls alignment ofliquid crystal molecules in the liquid crystal layer.
 5. The liquidcrystal display device according to claim 4, wherein the alignment filmis a horizontal alignment film that aligns the liquid crystal moleculesin the liquid crystal layer in a direction parallel to a surface of thealignment film.
 6. The liquid crystal display device according to claim4, wherein the alignment film is a photo-alignment film.
 7. The liquidcrystal display device according to claim 6, wherein the photo-alignmentfilm contains a polymer containing a cinnamate group.
 8. The liquidcrystal display device according to claim 6, wherein the photo-alignmentfilm contains a polymer with a structure derived from a cyclobutanering.
 9. A method for producing a liquid crystal display device thatincludes a liquid crystal layer and a substrate with a retardationlayer, the method comprising: forming a film that contains at least onetype of monomer including a photo-alignment monomer represented by thefollowing formula (1), and irradiating the film with polarized light toalign and polymerize molecules of the photo-alignment monomer andthereby forming the retardation layer, the formula (1) being:

wherein P¹ and P² are the same as or different from each other and eachrepresent an acrylic group or a methacrylic group; Sp¹ and Sp² are thesame as or different from each other and each represent —O—, —S—, —NH—,—COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or a direct bond; A¹ andA² are the same as or different from each other and each represent aC1-C12 linear, branched, or cyclic saturated alkylene group, a C1-C12linear, branched, or cyclic unsaturated alkylene group, or a directbond; and Z¹ represents —O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—,—NHCS—, —CSNH—, or a direct bond.
 10. The method for producing a liquidcrystal display device according to claim 9, wherein the method furtherincludes subjecting a surface of the retardation layer to rubbingtreatment and thereby causing the retardation layer to exhibit alignmentcontrol force to liquid crystal molecules in the liquid crystal layer.11. The method for producing a liquid crystal display device accordingto claim 9, wherein the method further includes forming on a surface ofthe retardation layer a polymer film that contains a polymer containinga cinnamate group, and irradiating the polymer film with polarized lightand thereby causing the polymer film to exhibit alignment control forceto liquid crystal molecules in the liquid crystal layer.
 12. The methodfor producing a liquid crystal display device according to claim 9,wherein the method further includes forming on a surface of theretardation layer a polymer film that contains a polymer containing acyclobutane ring, and irradiating the polymer film with polarized lightand thereby causing the polymer film to exhibit alignment control forceto liquid crystal molecules in the liquid crystal layer.
 13. A monomermaterial for a retardation layer, comprising at least one type ofmonomer including a photo-alignment monomer represented by the followingformula (1):

wherein P¹ and P² are the same as or different from each other and eachrepresent an acrylic group or a methacrylic group; Sp¹ and Sp² are thesame as or different from each other and each represent —O—, —S—, —NH—,—COO—, —OCO—, —NHCO—, —CONH—, —NHCS—, —CSNH—, or a direct bond; A¹ andA² are the same as or different from each other and each represent aC1-C12 linear, branched, or cyclic saturated alkylene group, a C1-C12linear, branched, or cyclic unsaturated alkylene group, or a directbond; and Z¹ represents —O—, —S—, —NH—, —COO—, —OCO—, —NHCO—, —CONH—,—NHCS—, —CSNH—, or a direct bond.